The present invention relates to the field of fiber optic based communications and measuring apparatus, and more particularly to light stimulated, oscillating resonant element apparatus for measuring physical parameters.
Certain definitions are required for clarity and to facilitate understanding of the present invention. As used herein, "radiant energy" includes both coherent and incoherent energy of a wavelength between 1000 and 100,000 Angstroms, and specifically including infrared, ultraviolet, and visible light energy. Such radiant energy may be described as "steady" or "continuous wave" ("CW") in order to distinguish it from radiant energy signals which are modified to carry information. "Modulation" is used broadly herein, and it is intended to mean a process of modifying some characteristics of a carrier so that it varies in step with the instantaneous value of another signal and, specifically, amplitude modulation. The "steady" radiant energy denominated herein refers to radiant energy having substantially constant intensity levels; i.e., absent short term variations in intensity and having a substantially unchanging spectral distribution. In referring to light signals which carry information, the terms "shuttered" and "interrupted" are used to refer to modulated light as well as the mechanism by which the modulation takes place. "Fluid" includes gases and/or liquids. The term "silvered" is used generically herein to describe a reflective metallization coating, or its equivalent. "Partially silvered" is used to describe such a coating having transmission and reflection characteristics, which may be high in ratio, one to the other. The term "force" is used to describe any physical parameter or phenomena capable of moving a body or modifying its motion, and specifically includes pressure and any parameter or phenomena capable of conversion to pressure. "Motor" is used in its broader sense, i.e., denominates a device that moves an object. The term "transducer" is used to describe a device to convert energy from one form to another, and as used herein, the terms "opto-electric transducer" and "electro-optic transducer" more specifically describe the class of devices useful for converting radiant energy to electrical energy, and electrical energy to radiant energy. "Cantilever beam" refers to that class of mechanical or other sensors in which a beam element is attached by one of its ends and which may be resonated. Such cantilever beam elements may be hollow, in which event they are denominated "resonant hollow beam" elements or structures.
As the advantages of fiber optic based communication and control of industrial processes become better known, increasing emphasis is being placed on various methods of simple, inexpensive, and reliable communication of low level radiant energy via fiber optics to the sensor site for making the desired measurement, and returning the measurement information on fiber optic paths to the control and measurement location. Among the many problems facing designers of such process control systems are the need for few, low light level optical paths and methods of accurately and reliably carrying out the measurements in such a way that the derived measurement information may be accurately communicated by means of fiber optic signals.
It is well known that the resonant frequency of a taut wire is a function of the tension on the wire. It is also recognized that a force measuring instrument can be based on this relationship, by causing the wire to vibrate while tensioned by an unknown force applied thereto and measuring the vibration frequency, as in U.S. Pat. No. 4,329,775. Similarly, it is known that by subjecting the interior of a vibrating hollow beam structure to pressure variations, the resonant frequency thereof is caused to vary in relation to the pressure variations. In the field of fiber optic technology, it is known that a vibrating element partially blocking the light pathway in a periodic manner between two aligned fiber optic elements will "shutter" the light passing along the second fiber optic element.
It is also known that a steady light beam can be directed down a first fiber optic element, modulated (for example, acoustically), and returned to a point adjacent its source via a second fiber optic element (U.S. Pat. Nos. 4,345,482 and 4,275,295). It has very recently been alleged that a vibrating wire element can be driven by sending a pulsating light down a first optical fiber element, passing a steady light beam down a second fiber optic element to a point where the vibrations modulate the steady light, and the frequency of the vibrations detected by reflecting the modulated light back along a third fiber optic element path. The modulation frequency might then be measured. Changed tension on the vibrating wire may cause the returned light energy to vary regularly with the tension on the wire [Jones, B. E. and G. S. Philp, "A Vibrating Wire Sensor with Optical Fibre Links for Force Measurement", Paper No. 05.1, Sensors and Their Applications, UMIST Manchester (UK) 20-22 Sept. 1983].
The proposal of Jones and Philp is illustrated in FIG. 2. In their suggested apparatus, thin wire is stretched between an anchor post and a pressure diaphragm. The wire is disposed between the poles of a permanent magnet and its two nodes are electrically connected, via a matching transformer, to photodiode 1. If light of alternating intensity is passed via an optical fiber to the photodiode, and an alternating current is driven through the wire, then it moves in a plane perpendicular to that defined by the current and the magnetic field at a frequency equal to that of the intensity fluctuations. The movement of the wire is sensed by two optical fibers placed in parallel and adjacent in the plane of the movement. The first of these two fibers is fed with light of nominally constant intensity from LED 2 in the control unit, and this light on leaving the fiber at the sensing head, illuminates the wire. Some light is reflected back into the second fiber and is returned to the control unit. The intensity of this returned light is a function of the position of the wire relative to the fiber ends. Therefore, as the wire vibrates, alternating light intensity in phase with the oscillation is returned to the control unit by the second fiber. This signal is electrically amplified in the control unit and a portion of the electric output used to drive LED 1 in phase and in resonance with the wire; a third optical fiber carries this in-phase light signal to the sensing head where it is converted into oscillatory driving power to sustain oscillation.
These and prior methods of remote detection and communication by fiber optic means generally require multiple light pathways, complex circuitry, and/or independent sources of oscillation energy for the resonant member.